Heating, ventilation and air conditioning (HVAC) systems typically exhaust a portion of the re-circulating air and simultaneously replace such exhaust air with fresh air. In order to maintain an air temperature and humidity level, within a certain space, at or near a set point, it is desirable to suitably condition the fresh air to a temperature below or above set point. Unfortunately, the temperature and humidity of fresh air often differ substantially from those of the set point. For example, during hot and humid periods, such as the summer months, the incoming fresh air typically has a higher temperature and/or humidity level than desired. Additionally, during cold and/or dry periods, such as the winter months, the incoming fresh air typically has a lower temperature and humidity level than desired. The HVAC system must, therefore, condition the fresh air before introducing it to the room.
HVAC systems are typically designed according to the worst climatic conditions for the geographic area in which the HVAC system will be located. Such worst case climatic conditions are referred to as a cooling or heating "design day." Conditioning the fresh air during such extreme climatic conditions creates a significant load on the HVAC system. System designers, therefore, typically design the HVAC system with sufficient capacity to maintain the set point during design day conditions. Such a HVAC system may include oversized equipment or include ventilators in order to operate effectively during such design day conditions. A ventilator typically includes an air-to-air heat exchanger, which creates alternating flow passages for the fresh air stream and exhaust air stream to pass therethrough, thereby transferring sensible and/or latent heat from one air stream to the other. Transferring heat between air streams reduces the load on the HVAC system and decreases its capacity requirements, which, in turn, allows the designers to specify lower capacity cooling or heating equipment, thereby leading to a more efficient design.
The air-to-air heat exchanger may be a plate-type heat exchanger or a cylindrical heat exchanger. Plate-type heat exchangers are typically constructed of a plurality of parallel plates that form alternating parallel or perpendicular passageways between such plates. If the alternating flow passages are perpendicular to one another, then the heat exchanger is referred to as a cross flow heat exchanger. Alternatively, if the flow passages are parallel to one another, then the heat exchanger is referred to as a co-flow or counter flow heat exchanger, depending upon the direction of the air streams. Counter flow heat exchangers are typically more efficient than cross flow heat exchangers. However, because the types of manifolds that are required to include a counter flow plate-type heat exchanger within a ventilator are typically complicated, most ventilators include cross flow plate-type heat exchangers. Thus, utilizing a counter flow plate-type heat exchanger may be more effective than a cross flow design, but the additional cost of the manifolding for the counter flow design may not justify the incremental improvement in performance.
Cylindrical heat exchangers are typically constructed of a plurality of annular passageways created by multiple welded circular layers that are concentric about the center of the cylindrical heat exchanger. Such layers typically create an efficient counter flow design in that one air stream enters one end and another air stream enters the other end and both air streams exit ends opposite those from which they entered the cylindrical heat exchanger. The annular passageways often include a continuous corrugated sheet therein. However, the continuous corrugated sheet could significantly decrease the pressure of the air stream as it passes through the passageway such that the resulting pressure drop of the air stream is undesirable. Moreover, the inclusion of the continuous corrugated sheet within the passageways could necessitate increasing the size of the HVAC's air handling equipment, along with its energy consumption, such that adding a ventilator to an HVAC system removes the cost benefit of including a ventilator within such a system.
Regardless of whether the heat exchanger is a plate-type or cylindrical heat exchanger, the ventilator is considered a heat recovery ventilator (HRV) or an energy recovery ventilator (ERV). Determining whether a ventilator is a HRV or an ERV is dependent upon the material from which the flat or circular plates are constructed. Moreover, such a determination is dependent upon whether the flat or circular plates are capable of transferring sensible heat or both sensible and latent heat. Specifically, if the plates or circular layers are constructed of a material that is only capable of transferring sensible heat, then the ventilator is referred to as a HRV. If, however, the plates or circular layers are constructed of a material that is capable of transferring latent heat, as well as sensible heat, then the ventilator is referred to as an ERV. For example, metal plates, such as aluminum plates, absorb a portion of the thermal energy in one air stream and transfer such energy to the other air stream by undergoing a temperature change without allowing any moisture to pass therethrough. Therefore, a ventilator constructed of metal plates is referred to as a HRV. Although plates constructed of paper typically have a lower thermal conductivity than metal, paper may be capable of transferring sensible heat because it is capable of transferring moisture between air streams. A ventilator having plates constructed of a material capable of transferring moisture between air streams is capable of transferring latent heat and is, therefore, referred to as an ERV.
It is generally understood that an ERV is more versatile and beneficial than an HRV. However, materials such as paper limit the plate's ability to transfer a larger portion of the latent heat from one air stream to the other air stream. Therefore, it is desirable to produce an ERV with a plate having a greater latent heat transfer capability. The cost of the more efficient material, however, cannot disrupt the cost benefit of including an ERV within a HVAC system. As discussed hereinbefore, utilizing a ventilator to pre-condition the fresh air permits selection of a lower capacity chiller or heater for the HVAC system. Specifically, pre-conditioning the fresh air allows the system designers to utilize a design day having more moderate parameters, which, in turn, make possible the inclusion of smaller, less costly equipment. Such equipment will also consume less energy, thereby making it less expensive to operate. Hence, including an ERV within a HVAC system is perceived as a low cost method for increasing the system's overall operating efficiency. However, if the cost of a more efficient plate material significantly increases the cost of the ERV, then including an ERV within a HVAC system decreases its financial benefit. Therefore, it is desirable that the plates within the plate-type heat exchanger be constructed of a low cost material, as well as a material that has the ability to effectively transfer latent heat.
What is needed is a cylindrical heat exchanger that minimizes the additional pressure drop of an HVAC system when such a heat exchanger is added to the system. Also, what is needed is a cylindrical heat exchanger having passageways separated by layers that are constructed of a cost effective material, other than paper, and that is capable of transferring a larger percentage of the available latent heat in one air stream to the other air stream.